Cyclic AMP-dependent protein kinase controls basal gene activity and steroidogenesis in Y1 adrenal tumor cells

Activation and expression of endogenous CREB-regulated transcription coactivators (CRTC) 1, 2 and 3 in the rat adrenal gland

The activation and nuclear translocation of cAMP-response element binding protein (CREB)-regulated transcription coactivator (CRTC)2 occurs in the rat adrenal gland, in response to adrenocorticotrophic hormone (ACTH) and stressors, and has been implicated in the transcriptional regulation of steroidogenic acute regulatory protein (StAR). We have recently demonstrated the activation of CRTC isoforms, CRTC1 and CRTC3, in adrenocortical cell lines. In the present study, we aimed to determine the activation and expression of the three CRTC isoforms in vivo in relation to Star transcription, under basal conditions and following a robust endotoxic stress challenge. Rat adrenal glands and blood plasma were collected following i.v. administration of either an ultradian-sized pulse of ACTH or administration of lipopolysaccharide, as well as under unstressed conditions across the 24-hour period. Plasma ACTH and corticosterone (CORT) were measured and the adrenal glands were processed for measurement of protein by western immunoblotting, RNA by a quantitative reverse transcriptase-polymerase chain reaction and association of CRTC2 and CRTC3 with the Star promoter by chromatin immunoprecipitation. An increase in nuclear localisation of CRTC2 and CRTC3 followed increases in both ultradian and endotoxic stress-induced plasma ACTH, and this was associated with increased CREB phosphorylation and corresponding increases in Star transcription. Both CRTC2 and CRTC3 were shown to associate with the Star promoter, with the dynamics of CRTC3 binding corresponding to that of nuclear changes in protein levels. CRTC isoforms show little variation in ultradian expression or variation across 24 hours, although evidence of long-term down-regulation following endotoxic stress was found. We conclude that co-transcription factors CRTC2 and, more clearly, CRTC3 appear to act alongside phosphorylated CREB in the generation of ultradian pulses of Star transcription, essential for the maintenance of basal StAR expression. Similarly, our findings suggest CRTC2 and CRTC3 mediate Star transcriptional initiation following an endotoxic stressor; however, other transcription factors are likely to be responsible for the long-term up-regulation of adrenal Star transcription.

Studies of mice with cyclic AMP-dependent protein kinase (PKA) defects reveal the critical role of PKA's catalytic subunits in anxiety

Cyclic adenosine mono-phosphate-dependent protein kinase (PKA) is critically involved in the regulation of behavioral responses. Previous studies showed that PKA's main regulatory subunit, R1α, is involved in anxiety-like behaviors. The purpose of this study was to determine how the catalytic subunit, Cα, might affect R1α's function and determine its effects on anxiety-related behaviors. The marble bury (MB) and elevated plus maze (EPM) tests were used to assess anxiety-like behavior and the hotplate test to assess nociception in wild type (WT) mouse, a Prkar1a heterozygote (Prkar1a(+/-)) mouse with haploinsufficiency for the regulatory subunit (R1α), a Prkaca heterozygote (Prkaca(+/-)) mouse with haploinsufficiency for the catalytic subunit (Cα), and a double heterozygote mouse (Prkar1a(+/-)/Prkaca(+/-)) with haploinsufficiency for both R1α and Cα. We then examined specific brain nuclei involved in anxiety. Results of MB test showed a genotype effect, with increased anxiety-like behavior in Prkar1a(+/-) and Prkar1a(+/-)/Prkaca(+/-) compared to WT mice. In the EPM, Prkar1a(+/-) spent significantly less time in the open arms, while Prkaca(+/-) and Prkar1a(+/-)/Prkaca(+/-) mice displayed less exploratory behavior compared to WT mice. The loss of one Prkar1a allele was associated with a significant increase in PKA activity in the basolateral (BLA) and central (CeA) amygdala and ventromedial hypothalamus (VMH) in both Prkar1a(+/-) and Prkar1a(+/-)/Prkaca(+/-) mice. Alterations of PKA activity induced by haploinsufficiency of its main regulatory or most important catalytic subunits result in anxiety-like behaviors. The BLA, CeA, and VMH are implicated in mediating these PKA effects in brain.

The Role of Protein Kinase A in Anxiety Behaviors

This review focuses on the genetic and other evidence supporting the notion that the cyclic AMP (cAMP) signaling pathway and its mediator, the protein kinase A (PKA) enzyme, which respond to environmental stressors and regulate stress responses, are central to the pathogenesis of disorders related to anxiety. We describe the PKA pathway and review in vitro animal studies (mouse) and other evidence that support the importance of PKA in regulating behaviors that lead to anxiety. Since cAMP signaling and PKA have been pharmacologically exploited since the 1940s (even before the identification of cAMP as a second messenger with PKA as its mediator) for a number of disorders from asthma to cardiovascular diseases, there is ample opportunity to develop therapies using this new knowledge about cAMP, PKA, and anxiety disorders.

Prostaglandin E(2)-dependent blockade of actomyosin and stress fibre formation is mediated through S1379 phosphorylation of ROCK2

Prostaglandin E2 is a pleiotropic bioactive lipid that controls cytoskeletal alterations, although the precise G-protein coupled EP receptor signalling mechanisms remain ill defined. We adopted a phosphoproteomic approach to characterize post-receptor downstream signalling substrates using antibodies that selectively recognize and immunoprecipitate phosphorylated substrates of a number of kinases. Using human synovial fibroblasts in monolayer cell culture, PGE2 induced rapid and sustained changes in cellular morphology and reduction in cytoplasmic volume that were associated with disassembly of the phalloidin-stained stress fibres as judged by light and confocal microscopy. Furthermore, PGE2 induced a rapid dephosphorylation of myosin light chain II (MLC) at S19 under basal or cytokine-induced conditions that was linked to an activation of myosin light chain phosphatase. The use of specific synthetic EP agonists suggested that the response was mediated by EP2 receptors, as other EP agonists did not manifest the same effect on MLC phosphorylation. In addition, PGE2 induced sustained Y118 dephosphorylation of phospho-paxillin and loss of focal adhesions as observed by confocal microscopy and Western analysis. Phosphoproteomic analysis of PGE2 /GPCR/PKA phosphosubstrates identified a unique, non-redundant, phosphorylated (>30-fold) site on rho-associated coiled coil-containing kinase 2 (ROCK2) at S1379. Analysis of ROCK2 mutant behaviour (e.g. S1379A) in overexpression studies revealed that PGE2 -dependent phosphorylation of ROCK2 resulted in the inhibition of the kinase, since induced MLC phosphorylation was no longer blocked by PGE2 nor could PGE2 induce disassembly of stress fibres. Thus, PGE2 -dependent blockade of actomyosin fibre formation, characteristic of myofibroblasts, may be mediated through specific ROCK2 S1379 phosphorylation.

The alternative Epac/cAMP pathway and the MAPK pathway mediate hCG induction of leptin in placental cells

Pleiotropic effects of leptin have been identified in reproduction and pregnancy, particularly in the placenta, where it works as an autocrine hormone. In this work, we demonstrated that human chorionic gonadotropin (hCG) added to JEG-3 cell line or to placental explants induces endogenous leptin expression. We also found that hCG increased cAMP intracellular levels in BeWo cells in a dose-dependent manner, stimulated cAMP response element (CRE) activity and the cotransfection with an expression plasmid of a dominant negative mutant of CREB caused a significant inhibition of hCG stimulation of leptin promoter activity. These results demonstrate that hCG indeed activates cAMP/PKA pathway, and that this pathway is involved in leptin expression. Nevertheless, we found leptin induction by hCG is dependent on cAMP levels. Treatment with (Bu)₂cAMP in combination with low and non stimulatory hCG concentrations led to an increase in leptin expression, whereas stimulatory concentrations showed the opposite effect. We found that specific PKA inhibition by H89 caused a significant increase of hCG leptin induction, suggesting that probably high cAMP levels might inhibit hCG effect. It was found that hCG enhancement of leptin mRNA expression involved the MAPK pathway. In this work, we demonstrated that hCG leptin induction through the MAPK signaling pathway is inhibited by PKA. We observed that ERK1/2 phosphorylation increased when hCG treatment was combined with H89. In view of these results, the involvement of the alternative cAMP/Epac signaling pathway was studied. We observed that a cAMP analogue that specifically activates Epac (CPT-OMe) stimulated leptin expression by hCG. In addition, the overexpression of Epac and Rap1 proteins increased leptin promoter activity and enhanced hCG. In conclusion, we provide evidence suggesting that hCG induction of leptin gene expression in placenta is mediated not only by activation of the MAPK signaling pathway but also by the alternative cAMP/Epac signaling pathway.

Cell-type specific expression of a dominant negative PKA mutation in mice

We employed the Cre recombinase/loxP system to create a mouse line in which PKA activity can be inhibited in any cell-type that expresses Cre recombinase. The mouse line carries a mutant Prkar1a allele encoding a glycine to aspartate substitution at position 324 in the carboxy-terminal cAMP-binding domain (site B). This mutation produces a dominant negative RIα regulatory subunit (RIαB) and leads to inhibition of PKA activity. Insertion of a loxP-flanked neomycin cassette in the intron preceding the site B mutation prevents expression of the mutant RIαB allele until Cre-mediated excision of the cassette occurs. Embryonic stem cells expressing RIαB demonstrated a reduction in PKA activity and inhibition of cAMP-responsive gene expression. Mice expressing RIαB in hepatocytes exhibited reduced PKA activity, normal fasting induced gene expression, and enhanced glucose disposal. Activation of the RIαB allele in vivo provides a novel system for the analysis of PKA function in physiology.

Regulation of placental leptin expression by cyclic adenosine 5'-monophosphate involves cross talk between protein kinase A and mitogen-activated protein kinase signaling pathways

Leptin, a 16-kDa protein mainly produced by adipose tissue, has been involved in the control of energy balance through its hypothalamic receptor. However, pleiotropic effects of leptin have been identified in reproduction and pregnancy, particularly in placenta, where it was found to be expressed. In the current study, we examined the effect of cAMP in the regulation of leptin expression in trophoblastic cells. We found that dibutyryl cAMP [(Bu)(2)cAMP], a cAMP analog, showed an inducing effect on endogenous leptin expression in BeWo and JEG-3 cell lines when analyzed by Western blot analysis and quantitative RT-PCR. Maximal effect was achieved at 100 microM. Leptin promoter activity was also stimulated, evaluated by transient transfection with a reporter plasmid construction. Similar results were obtained with human term placental explants, thus indicating physiological relevance. Because cAMP usually exerts its actions through activation of protein kinase A (PKA) signaling, this pathway was analyzed. We found that cAMP response element-binding protein (CREB) phosphorylation was significantly increased with (Bu)(2)cAMP treatment. Furthermore, cotransfection with the catalytic subunit of PKA and/or the transcription factor CREB caused a significant stimulation on leptin promoter activity. On the other hand, the cotransfection with a dominant negative mutant of the regulatory subunit of PKA inhibited leptin promoter activity. We determined that cAMP effect could be blocked by pharmacologic inhibition of PKA or adenylyl ciclase in BeWo cells and in human placental explants. Thereafter, we decided to investigate the involvement of the MAPK/ERK signaling pathway in the cAMP effect on leptin induction. We found that 50 microm PD98059, a MAPK kinase inhibitor, partially blocked leptin induction by cAMP, measured both by Western blot analysis and reporter transient transfection assay. Moreover, ERK 1/2 phosphorylation was significantly increased with (Bu)(2)cAMP treatment, and this effect was dose dependent. Finally, we observed that 50 microm PD98059 inhibited cAMP-dependent phosphorylation of CREB in placental explants. In summary, we provide some evidence suggesting that cAMP induces leptin expression in placental cells and that this effect seems to be mediated by a cross talk between PKA and MAPK signaling pathways.

PKA-dependent and independent cAMP signaling in 3T3-L1 fibroblasts differentiation

Adipogenesis is stimulated in 3T3-L1 fibroblast by a combination of insulin, dexamethasone, and methylisobutylxanthine (MIX). Mitotic clonal expansion (MCE) precedes differentiation of 3T3-L1 fibroblast to adipocytes. MIX increases cAMP content, which is the activator of protein kinase A (PKA). However, PKA-independent cAMP signaling has also been described. In this paper, it was found that H89, an inhibitor of PKA, was able to block MCE but not differentiation of 3T3-L1 fibroblast. Consistently, MCE did not occur in the absence of MIX in the differentiation mixture but was recovered by overexpression of a catalytic subunit of PKA. In addition, the transfection of 3T3-L1 fibroblast with a dominant-negative mutant of PKA inhibited MCE. On the other hand, differentiation of 3T3-L1 fibroblast to adipocytes did not occur when MIX was not present in the differentiation mixture and it could not be recovered by overexpression of a catalytic subunit of PKA. Differentiation was restored by addition of either dibutyryl-cAMP (db-cAMP) or 8 CPT-2 Me-cAMP. The latter activates cAMP-EPAC but not PKA signaling. These results indicate that cAMP-PKA-independent signaling, is required for 3T3-L1 fibroblasts differentiation to adipocytes and MIX signaling through cAMP-PKA is necessary for MCE, although MCE is not essential for adipogenesis.

Mechanisms of inhibition of dehydroepiandrosterone upon corticosterone release from rat zona fasciculata-reticularis cells

We have demonstrated that dehydroepiandrosterone (DHEA) acts directly on rat zona fasciculata-reticularis (ZFR) cells to diminish corticosterone secretion by an inhibition of post-cAMP pathway, and decreases functions of steroidogenic enzymes after P450(scc) as well as steroidogenic acute regulatory (StAR) protein expression. However, the mechanisms by which DHEA engages with environmental messenger signals which translate into interfering StAR protein expression are still unclear. This study explored the effects of DHEA on the phosphorylation/activation of extracellular signal-regulated kinases (ERKs). ERK activation resulted in enhancing phosphorylation of steroidogenic factor-1 (SF-1) and increased StAR protein expression. ZFR cells were incubated in the presence or absence of adrenocorticotropin (ACTH), forskolin (FSK), 25-OH-cholesterol, U0126, and H89 at 37 degrees C. The concentration of corticosterone released into the media was measured by radioimmunoassay (RIA). The cells were used to extract protein for Western blot analysis of ERKs or StAR protein expression or immunoprecipitation of SF-1 analysis. The results suggested that (1) ERK pathway of rat ZFR cells might be PKA dependent, (2) ERK activity was required for SF-1 phosphorylation to upregulate steroidogenesis in rat ZFR cells, and (3) DHEA did not affect ERK phosphorylation, however, it attenuated forskolin-stimulated SF-1 phosphorylation to affect StAR protein expression.

Delineation of Type I Protein Kinase A-selective Signaling Events Using an RI Anchoring Disruptor

Control of specificity in cAMP signaling is achieved by A-kinase anchoring proteins (AKAPs), which assemble cAMP effectors such as protein kinase A (PKA) into multiprotein signaling complexes in the cell. AKAPs tether the PKA holoenzymes at subcellular locations to favor the phosphorylation of selected substrates. PKA anchoring is mediated by an amphipathic helix of 14-18 residues on each AKAP that binds to the R subunit dimer of the PKA holoenzymes. Using a combination of bioinformatics and peptide array screening, we have developed a high affinity-binding peptide called RIAD (RI anchoring disruptor) with >1000-fold selectivity for type I PKA over type II PKA. Cell-soluble RIAD selectively uncouples cAMP-mediated inhibition of T cell function and inhibits progesterone synthesis at the mitochondria in steroid-producing cells. This study suggests that these processes are controlled by the type I PKA holoenzyme and that RIAD can be used as a tool to define anchored type I PKA signaling events.

Global profiles of gene expression induced by adrenocorticotropin in Y1 mouse adrenal cells

ACTH regulates the steroidogenic capacity, size, and structural integrity of the adrenal cortex through a series of actions involving changes in gene expression; however, only a limited number of ACTH-regulated genes have been identified, and these only partly account for the global effects of ACTH on the adrenal cortex. In this study, a National Institute on Aging 15K mouse cDNA microarray was used to identify genome-wide changes in gene expression after treatment of Y1 mouse adrenocortical cells with ACTH. ACTH affected the levels of 1275 annotated transcripts, of which 46% were up-regulated. The up-regulated transcripts were enriched for functions associated with steroid biosynthesis and metabolism; the down- regulated transcripts were enriched for functions associated with cell proliferation, nuclear transport and RNA processing, including alternative splicing. A total of 133 different transcripts, i.e. only 10% of the ACTH-affected transcripts, were represented in the categories above; most of these had not been described as ACTH-regulated previously. The contributions of protein kinase A and protein kinase C to these genome-wide effects of ACTH were evaluated in microarray experiments after treatment of Y1 cells and derivative protein kinase A-defective mutants with pharmacological probes of each pathway. Protein kinase A-dependent signaling accounted for 56% of the ACTH effect; protein kinase C-dependent signaling accounted for an additional 6%. These results indicate that ACTH affects the expression profile of Y1 adrenal cells principally through cAMP- and protein kinase A- dependent signaling. The large number of transcripts affected by ACTH anticipates a broader range of actions than previously appreciated.

Adrenocorticotropic Hormone-Mediated Signaling Cascades Coordinate a Cyclic Pattern of Steroidogenic Factor 1-Dependent Transcriptional Activation

Steroidogenic factor 1 (SF-1) is an orphan nuclear receptor that has emerged as a critical mediator of endocrine function at multiple levels of the hypothalamic-pituitary-steroidogenic axis. Within the adrenal cortex, ACTH-dependent transcriptional responses, including transcriptional activation of several key steroidogenic enzymes within the steroid biosynthetic pathway, are largely dependent upon SF-1 action. The absence of a bona fide endogenous eukaryotic ligand for SF-1 suggests that signaling pathway activation downstream of the melanocortin 2 receptor (Mc2r) modulates this transcriptional response. We have used the chromatin immunoprecipitation assay to examine the temporal formation of ACTH-dependent transcription complexes on the Mc2r gene promoter. In parallel, ACTH-dependent signaling events were examined in an attempt to correlate transcriptional events with the upstream activation of signaling pathways. Our results demonstrate that ACTH-dependent signaling cascades modulate the temporal dynamics of SF-1-dependent complex assembly on the Mc2r promoter. Strikingly, the pattern of SF-1 recruitment and the subsequent attainment of active rounds of transcription support a kinetic model of SF-1 transcriptional activation, a model originally established in the context of ligand-dependent transcription by several classical nuclear hormone receptors. An assessment of the major ACTH-dependent signaling pathways highlights pivotal roles for the MAPK as well as the cAMP-dependent protein kinase A pathway in the entrainment of SF-1-mediated transcriptional events. In addition, the current study demonstrates that specific enzymatic activities are capable of regulating distinct facets of a highly ordered transcriptional response.

Adrenocortical cell lines

The human adrenal cortex is a complex endocrine organ that secretes mineralocorticoids, glucocorticoids and adrenal androgens. These steroids arise from morphologically and biochemically distinct zones of the adrenal gland. Studying secretion of these distinct steroid hormones can make use of cells isolated from the adrenal gland but this requires animal sacrifice and the need for continued isolation for long-term studies. In addition primary cultures of adrenal cells have a limited life-span in culture and the cultured cells are often contaminated by the presence of non-steroidogenic cells. For that reason in vitro cell culture models have several benefits for research on adrenocortical function. Herein we discuss the available adrenocortical cell lines and their uses as model systems for adrenal studies. Focus is placed on the human NCI-H295 and mouse Y-1 adrenal cell lines, which have been used extensively as adrenocortical model systems. These cell lines have proven to be of considerable value in studying the molecular and biochemical mechanisms controlling adrenal steroidogenesis. The current review will discuss the attributes and limitations of the currently available adrenocortical cell lines as models for adrenal studies.

Effects of S-petasin on cyclic AMP production and enzyme activity of P450scc in rat zona fasciculata-reticularis cells

Petasites hybridus is used in Chinese herbal medicine. S-petasin is a bioactive compound isolated from leaves or roots of P. hybridus, which has been used to relieve gastrointestinal pain, lung disease, and spasms of urogenital tract. We have demonstrated that S-petasin inhibited corticosterone release from rat zona fasiculata-reticularis cells. However, the mechanism and molecular effects of S-petasin on zona fasiculata-reticularis cells are still unclear. This study explored the effects of S-petasin on cellular adenosine 3':5'-cyclic monophosphate (cAMP) production, the functions of steroidogenic enzymes including cytochrome P450 side-chain cleavage enzyme (P450scc), 11beta-hydroxylase, and the expression levels of steroidogenic acute regulatory protein or P450scc. In this experiment, zona fasciculata-reticularis cells were incubated with S-petasin in the presence or absence of adrenocorticotropin (ACTH), 8-bromo-adenosine 3':5'-cyclic monophosphate (8-Br-cAMP), forskolin, 25-OH-cholesterol, deoxycorticosterone at 37 degrees C for 0.5, 1 or 3 h. The media were used to measure the concentration of corticosterone or pregnenolone by radioimmunoassay. The cells were used to measure the content of cAMP by radioimmunoassay and extracted protein for Western blot or messenger RNA (mRNA) for reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Our data demonstrated that (1) S-petasin inhibits ACTH- or forskolin-stimulated cellular cAMP production, (2) S-petasin increased the Michaelis constants of P450scc and 11beta-hydroxylase and (3) S-petasin decreased the expression levels and mRNA of steroidogenic acute regulatory protein. In summary, the actions of S-petasin mediate the inhibition of cAMP formation, decrease the activities of key enzymes P450scc and 11beta-hydroxylase, and reduce mRNA of steroidogenic acute regulatory protein and expression of steroidogenic acute regulatory protein.

Effects of dehydroepiandrosterone on corticosterone release in rat zona fasciculata-reticularis cells

The decline of plasma dehydroepiandrosterone (DHEA) and maintenance of glucocorticoid levels with increasing age contribute to excess body fat accumulation, hyperglycaemia, hyperlipidaemia, hyperinsulinaemia and cancer. Although opposing actions of DHEA and corticosterone have been proposed in a rat model, the effects and action mechanisms of DHEA on rat adrenal zona fasciculata-reticularis (ZFR) cells are still unclear. This study addressed the effects of DHEA on corticosterone release, cellular cAMP production, the functions of steroidogenic enzymes and the expression levels of steroidogenic acute regulatory protein (StAR) and cytochrome P450 side-chain cleavage enzyme (P450scc). ZFR cells were incubated with DHEA in the presence or absence of adrenocorticotropin (ACTH), 8-Br-cAMP, forskolin, 25-OH-cholesterol, pregnenolone, progesterone or deoxycorticosterone at 37 degrees C for 30 min, 1 h or 5 h and the concentration of corticosterone or pregnenolone measured subsequently in the media by RIA. The cells were used to measure the content of cAMP by RIA and to extract protein for Western blot or mRNA for RT-PCR analysis. The data demonstrated that (1) DHEA inhibited ACTH-, 8-Br-cAMP-, 25-OH-cholesterol-, pregnenolone-, progesterone- or deoxycorticosterone-stimulated corticosterone release; (2) DHEA increased 25-OH-cholesterol-stimulated pregnenolone release but not when 25-OH-cholesterol was combined with trilostane; (3) DHEA increased the K(m) of 11beta-hydroxylase but not P450scc; (4) DHEA affected the expression levels of StAR protein but not of P450scc. These results suggest that DHEA acts directly on rat ZFR cells to diminish corticosterone secretion by inhibition within the post-cAMP pathway, by inhibiting steroidogenic enzymes downstream from P450scc and by inhibiting StAR expression.

Increased basal cAMP-dependent protein kinase activity inhibits the formation of mesoderm-derived structures in the developing mouse embryo

A targeted disruption of the RIalpha isoform of protein kinase A (PKA) was created by using homologous recombination in embryonic stem cells. Unlike the other regulatory and catalytic subunits of PKA, RIalpha is the only isoform that is essential for early embryonic development. RIalpha homozygous mutant embryos fail to develop a functional heart tube at E8.5 and are resorbed at approximately E10.5. Mutant embryos show significant growth retardation and developmental delay compared with wild type littermates from E7.5 to E10.5. The anterior-posterior axis of RIalpha mutants is well developed, with a prominent head structure but a reduced trunk. PKA activity measurements reveal an increased basal PKA activity in these embryos. Brachyury mRNA expression in the primitive streak of RIalpha mutants is significantly reduced, consistent with later deficits in axial, paraxial, and lateral plate mesodermal derivatives. This defect in the production and migration of mesoderm can be completely rescued by crossing RIalpha mutants to mice carrying a targeted disruption in the Calpha catalytic subunit, demonstrating that unregulated PKA activity rather than a specific loss of RIalpha is responsible for the phenotype. Primary embryonic fibroblasts from RIalpha mutant embryos display an abnormal cytoskeleton and an altered ability to migrate in cell culture. Our results demonstrate that unregulated PKA activity negatively affects growth factor-mediated mesoderm formation during early mouse development.

Activation and induction of NUR77/NURR1 in corticotrophs by CRH/cAMP: involvement of calcium, protein kinase A, and MAPK pathways

Nur factors are critical for proopiomelanocortin (POMC) induction by CRH in corticotrophs, but the pathways linking CRH to Nur are unknown. In this study we show that in AtT-20 corticotrophs CRH and cAMP induce Nur77 and Nurr1 expression and transcription at the NurRE site by protein kinase A (PKA) and calcium-dependent and -independent mechanisms. Calcium pathways depend on calmodulin kinase II (CAMKII) activity, and calcium-independent pathways are accounted for in part by MAPK activation (Rap1/B-Raf/MAPK-ERK kinase/ERK1/2), demonstrated by the use of molecular and pharmacological tools. AtT-20 corticotrophs express B-Raf, as do other cells in which cAMP stimulates MAPK. CRH/cAMP stimulated ERK2 activity and increased transcriptional activity of a Gal4-Elk1 protein, which was blocked by overexpression of dominant negative mutants and kinase inhibitors and stimulated by expression of B-Raf. The MAPK kinase inhibitors did not affect Nur77 and Nurr1 mRNA induction but blocked CRH or cAMP-stimulated Nur transcriptional activity. Moreover, MAPK stimulated phosphorylation and transactivation of Nur77. The functional impact of these pathways was confirmed at the POMC promoter. In conclusion, in AtT-20 corticotrophs the CRH/cAMP signaling that leads to Nur77/Nurr1 mRNA induction and transcriptional activation, and thus POMC expression, is dependent on protein kinase A and involves calcium/calmodulin kinase II (Nur induction/activation) and MAPK calcium-dependent and -independent (Nur phosphorylation-activation) pathways.

Steroidogenic acute regulatory protein expression is dependent upon post-translational effects of cAMP-dependent protein kinase A

Tropic hormones acutely stimulate adrenal and gonadal steroidogenesis by activation of the cAMP-dependent protein kinase A (PKA) signaling pathway and subsequent induction of Steroidogenic Acute Regulatory (StAR) protein (StAR) expression. We present a comparative study of StAR regulation in mouse adrenocortical Y1 and the derived PKA mutant Kin-8 cell lines to evaluate the PKA requirement for StAR expression. A parallel increase in StAR steady-state mRNA and protein was observed in Y1 cells. StAR mRNA was induced in 8-Br-cAMP-treated Kin-8 cells with maximal expression levels approx. 50% of that observed in Y1 cells. However, a corresponding increase in StAR protein, as detected by Western analysis, was absent in the Kin-8 cells. A similar distribution of StAR mRNA in active polysome fractions was observed for both 8-Br-cAMP-treated Y1 and Kin-8 cells, as well as a 2-fold increase in incorporation of [35S]methionine into StAR, which indicated translation was not blocked in Kin-8 cells. Together these data indicate that PKA functions at the post-translational level to regulate StAR expression and we propose that phosphorylation of StAR by PKA contributes to protein stability

Proline transport in MDCK cells expressing a mutant regulatory subunit of cAMP-dependent protein kinase

cAMP-dependent protein kinase (cAK) regulates the activity of several membrane-bound ion channels and carriers. The role of cAK in regulating the transport of osmoprotective amino acids in the distal tubule is unknown. We examined the regulation of Na(+)- and Cl(-)-dependent proline transport in MDCK cells expressing a mutant murine regulatory subunit (RIalpha(AB)) of cAK. For this purpose, MDCK cells were transfected with an expression vector encoding RIalpha(AB) driven by the metallothionein 1 promoter together with neomycin-resistance (NEO) gene. Stable G418-resistant colonies were isolated that expressed RIalpha(AB) as demonstrated by Northern hybridization analysis using a cDNA probe for RIalpha and cAK assay that showed decreased enzyme activity. A clone constitutively expressing high levels of RIalpha(AB) (M(AB)) in a Zn-independent manner and a control clone transfected with the NEO gene alone (M(neo)) were selected for transport studies. We examined the effect of the cAMP-stimulating agents forskolin (F) and IBMX on NaCl-dependent uptake of [(3)H]proline by confluent monolayers of transfected MDCK cells. While F/IBMX-induced mean inhibition of proline transport in M(neo) cells was 48 and 45% at 5 and 15 min, respectively, inhibition of proline uptake in M(AB) cells was 9% (5 min) and 0% (15 min). These data demonstrate that the inhibition of NaCl-linked proline transport in response to elevated cAMP is reversed in MDCK clones that express mutant cAK and provide evidence that cAK mediates the modulatory action of cAMP on proline transport. cAK may play an important role in controlling transport of proline and other osmoprotective amino acids in the renal tubule.

Post-translational regulation of steroidogenic acute regulatory protein by cAMP-dependent protein kinase A

Adrenal steroid production is stimulated by adrenocorticotropin hormone activation of the cAMP-dependent protein kinase A (PKA) signaling pathway and subsequent induction of Steroidogenic Acute Regulatory (StAR) protein expression. Herein we have compared StAR mRNA and protein levels in 8-Br-cAMP-treated mouse adrenocortical Y1 and the derived PKA mutant Kin-8 cell lines to evaluate the PKA requirement in StAR expression. StAR mRNA was induced by 8-Br-cAMP-treatment of both Y1 and Kin-8 cells with maximal expression levels in Kin-8 cells approximately 50% of that observed in Y1 cells. StAR protein levels, as detected by Western analysis, were concomitantly increased in Y1 cells but were not detected in the Kin-8 cells. StAR mRNA colocalized with the active polysome fractions in both 8-Br-cAMP-treated Y1 and Kin-8 cells, indicating translation was not blocked in Kin-8 cells. Consistent with this data, a 2-fold increase in incorporation of [35S]methionine into StAR was also observed after 8-Br-cAMP treatment of both cell lines. Since StAR protein levels were not sufficient to detect by Western analysis, these data indicate that PKA functions at the post-translational level to regulate StAR expression and we propose that phosphorylation of StAR by PKA contributes to protein stability.

Transcriptional regulation of 5-aminolevulinate synthase by phenobarbital and cAMP-dependent protein kinase

5-Aminolevulinate synthase (ALA-S) is a mitochondrial matrix enzyme that catalyzes the first and rate-limiting step of the heme biosynthesis. There are two ALA-S isozymes encoded by distinct genes. One gene encodes an isozyme that is expressed exclusively in erythroid cells, and the other gene encodes a housekeeping isozyme that is apparently expressed in all tissues. In this report we examine the mechanisms by which phenobarbital and cAMP regulate housekeeping ALA-S expression. We have determined that cAMP and phenobarbital effects are additive and the combined action is necessary to observe the cAMP effect on ALA-S mRNA in rat hepatocytes. The role of the cAMP-dependent protein kinase (PKA) has been examined. A synergism effect on ALA-S mRNA induction is observed in rat hepatocytes treated with pairs of selective analogs by each PKA cAMP binding sites. A 870-bp fragment of ALA-S 5'-flanking region is able to provide cAMP and phenobarbital stimulation to chloramphenicol O-acetyltranferase fusion vectors in transiently transfected HepG2 cells. ALA-S promoter activity is induced by cotransfection with an expression vector containing the catalytic subunit of PKA. Furthermore, cotransfection with a dominant negative mutant of the PKA regulatory subunit impairs the cAMP analog-mediated increase, but the phenobarbital-mediated induction is not modified. Our data suggest that the transcription factor cAMP-response element binding protein (CREB) is probably involved in PKA induction of ALA-S gene expression. Finally, heme addition greatly decreases the basal and phenobarbital or cAMP analog-mediated induction of ALA-S promoter activity. The present work provides evidence that cAMP, through PKA-mediated CREB phosphorylation, and phenobarbital induce ALA-S expression at the transcriptional level, while heme represses it.

Role of genomic mechanisms on cAMP-dependent positive inotropism in isolated left atrium of rat

It is well known that beta-adrenoceptor stimulation induces positive inotropism by cAMP-dependent phosphorylation of cardiac calcium channels. Furthermore, hypertrophy of different tissues including the heart have been related to the stimulation of these adrenoceptors via mechanisms coupled to activation of transcription and protein synthesis. Early effects of isoproterenol mediated via this pathway has also been associated to the stimulation of beta-adrenoceptors. However, the effects on the inotropism through genomic mechanisms have not yet been described. Isoproterenol (3 nM to 3 microM) induced a concentration-dependent positive inotropism, in isolated left atrium of male Wistar rats electrically stimulated (0.5 Hz, 5 ms, 30-50% above the threshold voltage), which was antagonized by atenolol (1 microM) and inhibited by a protein kinase A inhibitor, (R)p-cAMPS (10 microM). The inhibitor of transcription, actinomycin D (4 microM), and the protein synthesis inhibitor, cycloheximide (35.5 microM), significantly decreased the positive inotropism induced by isoproterenol. Forskolin (0.1 to 3 microM), an activator of adenylyl cyclase, induced a concentration-dependent positive inotropism which was also inhibited by (R)p-cAMPS, actinomycin D and cycloheximide. In the left atrium of rat, isoproterenol induced a positive inotropism which seems, at least in part, dependent upon intact transcription and protein synthesis, as suggested by the fact that the response was inhibited by the incubation with actinomycin D and cycloheximide. In addition, this genomic effect seems to be mediated by a cAMP-dependent mechanism. As it was inhibited by a protein kinase A inhibitor ((R)p-cAMPS) and similarly to isoproterenol, the positive inotropism induced by forskolin, which increases cytosolic cAMP, was also inhibited by actinomycin D and cycloheximide.

Calcium ion as a second messenger for o-nitrophenylsulfenyl-adrenocorticotropin (NPS-ACTH) and ACTH in bovine adrenal steroidogenesis

o-Nitrophenyl sulfenyl-modified ACTH (NPS-ACTH) stimulated steroidogenesis acutely in bovine fasciculata-reticularis cells without increase in cellular cAMP synthesis. Application of NPS-ACTH to the cultured cells induced Ca2+ signals in individual cells as detected by video-enhanced microscopic fluorescence measurements. The percentage of Ca2+ signaling cells corresponded well with the increase of steroidogenesis induced by NPS-ACTH below 1 nM. Treatment of the cells with nicardipine, a Ca2+ channel blocker, suppressed the Ca2+ signals except for the transient increase just after the addition of NPS-ACTH and also blocked completely the stimulative effect on the steroidogenesis of NPS-ACTH below 1 nM. At a dosage of NPS-ACTH higher than 10 nM, the stimulative effect of steroidogenesis was partly suppressed by nicardipine and also by AA-861, a lipoxygenase inhibitor. The action of NPS-ACTH might be mediated by both Ca2+ and lipoxygenase metabolite(s) of arachidonic acid as dual second messengers. The effect of ACTH in pM range on the steroidogenesis was suppressed completely by the treatment with nicardipine and AA-861 at the same time, indicating that the action was mediated by both Ca2+ and the lipoxygenase metabolite(s) but not by cAMP. cAMP plays a significant role as a second messenger for ACTH action only at ACTH concentrations greater than 10 pM.

Pharmacological evidence for the contribution of polyamines as mediators of the transcriptional component involved in smooth muscle relaxation elicited by forskolin

To study whether cAMP-dependent transcriptional effect and polyamines might play a modulatory role on smooth muscle, the effect of forskolin on KCl (60 mM)-induced contractions in isolated rat uterus and its modification by inhibitors of cAMP-dependent protein kinase (PKA) (Rp-cAMPS and TPCK), transcription (actinomycin D), protein synthesis (cycloheximide) and ornithine decarboxylase (alpha-difluoromethyl-ornithine, DFMO), and a polyamine (spermine) have been assayed. Forskolin (0.1 to 6 microM) induced concentration-dependent relaxation on KCl-induced tonic contractions in rat uterus (IC50: 0.55 +/- 0.12 microM) which was antagonized (p<0.05) by Rp-cAMPS (30 microM), TPCK (3 microM), cycloheximide (300 microM), actinomycin D (4 and 12 microM) and TPCK (3 microM) plus actinomycin D (12 microM). The IC50 values of forskolin in the presence of these drugs were 3.75 +/- 1.53 microM, 12.08 +/- 8.18 microM, 6.88 +/- 5.02 microM, 3.80 +/- 2.35 and 5.31 +/- 2.80 microM, and 4.26 +/- 3.65 microM respectively. Furthermore, DFMO (10 mM) also shifted the relaxation curve to forskolin to the right (IC50: 3.06 +/- 2.66 microM, p<0.05) but DFMO (10 mM) plus actinomycin D (12 microM) (IC50: 1.78 +/- 1.33 microM) did not. However, DFMO (10 mM) and actinomycin D (12 microM) did not antagonize the spermine (1-30 mM)-elicited relaxation (IC50s: 7.8 +/- 0.7 mM vs 7.28 +/- 1.4 mM and 4.67 +/- 0.44 mM in the presence of DFMO and actinomycin D, respectively). Moreover, spermine (1 mM) did not decrease the forskolin induced relaxation and counteracted the antagonism produced by actinomycin D and DFMO. Our results suggest that, in rat uterus, forskolin: a) produced cAMP-dependent relaxation, as this is antagonized by Rp-cAMP and TPCK, and b) increased the activity of ornithine decarboxylase, as this is inhibited by DFMO. Therefore, polyamines could be the mediator of the cAMP-dependent transcriptional component involved in forskolin relaxation, since, as mentioned, DFMO antagonized this relaxation and spermine counteracted the displacement produced by DFMO and actinomycin D. Thus, a plasma membrane-nucleus interaction might, at least partially, explain the mechanisms involved in forskolin induced relaxation in smooth muscle of rat uterus under the present experimental conditions.

PKA inhibitor, H-89, affects the intracellular transit of regulated secretory proteins in rat lacrimal glands

We tested the effect of H-89, a protein kinase A (PKA) inhibitor, on the intracellular transit of the regulated secretory proteins in rat lacrimal glands. We show that H-89, by itself, induces the secretion of newly synthesized proteins trafficking in its presence but not of proteins already stored in the mature secretory granules. This secretion does not depend on the presence of extracellular Ca2+. The proteins released are identical to those secreted after cholinergic stimulation or under the action of the ionophore A-23187, but the secretion level is approximately 40% lower. The effect of H-89 seems to be due to PKA inhibition because other protein kinase inhibitors (calphostin C, chelerythrine, H-85) do not induce secretion. We further show that H-89 does not modify the rate of glycoprotein galactosylation but induces the secretion of newly galactosylated glycoproteins. Finally, we used a "20 degrees C block" procedure to show that H-89 affects a trans-Golgi network (TGN) or post-TGN step of the secretory pathway. Our results demonstrate that, in lacrimal cells, H-89 affects the intracellular trafficking of secretory proteins, suggesting a role for PKA in this process.

The activity of neurogenin1 is controlled by local cues in the zebrafish embryo

Zebrafish neurogenin1 encodes a basic helix-loop-helix protein which shares structural and functional characteristics with proneural genes of Drosophila melanogaster. neurogenin1 is expressed in the early neural plate in domains comprising more cells than the primary neurons known to develop from these regions and its expression is modulated by Delta/Notch signalling, suggesting that it is a target of lateral inhibition. Misexpression of neurogenin1 in the embryo results in development of ectopic neurons. Markers for different neuronal subtypes are not ectopically expressed in the same patterns in neurogenin1-injected embryos suggesting that the final identity of the ectopically induced neurons is modulated by local cues. Induction of ectopic motor neurons by neurogeninl requires coexpression of a dominant negative regulatory subunit of protein kinase A, an intracellular transducer of hedgehog signals. Moreover, the pattern of endogenous neurogenin1 expression in the neural plate is expanded in response to elevated levels of Hedgehog (Hh) signalling or abolished as a result of inhibition of Hh signalling. Together these data suggest that Hh signals regulate neurogenin1 expression and subsequently modulate the type of neurons produced by Neurogenin1 activity.

Expression and regulation of a netrin homologue in the zebrafish embryo

Proteins of the Netrin family have been implicated in axon guidance in both C. elegans and vertebrates. Here, we report the cloning and expression analysis of a zebrafish netrin homologue (net1). net1 is expressed in the floor plate and the anterior ventral neural tube. Its expression is ectopically induced by misexpression of sonic hedgehog (shh) and a dominant negative mutant of the regulatory subunit of protein kinase A (dnReg). Ectopic activation of net1, however, is restricted to distinct regions in the brain. Upon overexpression of shh or dnReg in cyclops mutants, which have strongly impaired net1 expression in the ventral neural tube, rescue of net1 expression was observed in the brain but not in the spinal cord. Ectopic expression of dnReg and Shh protein can be detected at high levels throughout injected embryos from pre-gastrula stages onwards suggesting that the competence of the neural plate to respond to Shh signalling activity differs regionally. Similar to net1, axial, the zebrafish homologue of mammalian HNF3beta, which is also expressed along the ventral neural tube, is ectopically induced in the brain of embryos injected with dnReg mRNA. Neurons differentiate normally within domains of ectopic net1 and axial expression. Thus, dorsal neuronal differentiation appears to be unaffected despite co-expression of a gene program specific for the ventral neural tube. This also suggests that these ectopically expressing regions have not differentiated into floor plate.

The major catalytic subunit isoforms of cAMP-dependent protein kinase have distinct biochemical properties in vitro and in vivo

Two isoforms of the catalytic subunit of cAMP-dependent protein kinase, Calpha and Cbeta1, are known to be widely expressed in mammals. Although much is known about the structure and function of Calpha, few studies have addressed the possibility of a distinct role for the Cbeta proteins. The present study is a detailed comparison of the biochemical properties of these two isoforms, which were initially expressed in Escherichia coli and purified to homogeneity. Cbeta1 demonstrated higher Km values for some peptide substrates than did Calpha, but Cbeta1 was insensitive to substrate inhibition, a phenomenon that was observed with Calpha at substrate concentrations above 100 microM. Calpha and Cbeta1 displayed distinct IC50 values for the alpha and beta isoforms of the protein kinase inhibitor, protein kinase inhibitorpeptide, and the type IIalpha regulatory subunit (RIIalpha). Of particular interest, purified type II holoenzyme containing Cbeta1 exhibited a 5-fold lower Ka value for cAMP (13 nM) than did type II holoenzyme containing Calpha (63 nM). This latter result was extended to in vivo conditions by employing a transcriptional activation assay. In these experiments, luciferase reporter activity in COS-1 cells expressing RIIalpha2Cbeta12 holoenzyme was half-maximal at 12-fold lower concentrations of 8-(4-chlorophenylthio)-cAMP and 5-fold lower concentrations of forskolin than in COS-1 cells expressing RIIalpha2Calpha2 holoenzyme. These results provide evidence that type II holoenzyme formed with Cbeta1 is preferentially activated by cAMP in vivo and suggest that activation of the holoenzyme is determined in part by interactions between the regulatory and catalytic subunits that have not been described previously.

Basal expression of the cystic fibrosis transmembrane conductance regulator gene is dependent on protein kinase A activity

The cystic fibrosis transmembrane conductance regulator (CFTR) functions as a Cl- channel that becomes activated after phosphorylation by cAMP-dependent protein kinase (PKA). We demonstrate that PKA also plays a crucial role in maintaining basal expression of the CFTR gene in the human colon carcinoma cell line T84. Inhibition of PKA activity by expression of a dominant-negative regulatory subunit or treatment with the PKA-selective inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89) caused a complete suppression of CFTR gene expression without affecting other constitutively active genes. Basal expression of a 2.2-kb region of the CFTR promoter linked to a luciferase reporter gene (CFTR-luc) exhibited the same dependence on PKA. The ability of cAMP to induce CFTR over basal levels is cell-type specific. In T84 cells, both the endogenous CFTR gene and CFTR-luc exhibited only a modest inducibility (approximately 2-fold), whereas in the human choriocarcinoma cell line JEG-3, CFTR-luc could be induced at least 4-fold. A variant cAMP-response element is present at position -48 to -41 in the CFTR promoter, and mutation of this sequence blocks basal expression. We conclude that cAMP, acting through PKA, is an essential regulator of basal CFTR gene expression and may mediate an induction of CFTR in responsive cell types.

Expression of Chinese hamster cAMP-dependent protein kinase in Escherichia coli results in growth inhibition of bacterial cells: a model system for the rapid screening of mutant type I regulatory subunits

The regulatory and catalytic subunits of cAMP-dependent protein kinase (PKA) were coexpressed within the same bacterial cell using a polycistronic bacterial T7 expression vector encoding Chinese hamster cDNAs for the type I regulatory (RI) and catalytic alpha (C alpha) subunits of PKA. Basal expression of active RI/C alpha holoenzyme in the BL21(DE3) strain of Escherichia coli caused severe growth inhibition resulting in extremely small colony size. Several lines of evidence demonstrate that this growth inhibition requires active PKA subunits and cAMP: (i) this phenotype is dependent on cAMP since it is not seen in a strain lacking adenylyl cyclase activity, but the growth rate of these transformants is slower when exogenous cAMP is added; (ii) normal growth occurs when wild-type RI cDNA is replaced by a mutant RI cDNA encoding a RI protein with reduced cAMP binding; and (iii) the growth-inhibited phenotype of the transformed BL21(DE3) cells requires soluble, active C alpha protein. Holoenzyme expressed in bacteria is activated by cAMP, which stimulates phosphorylation of an endogenous 50-kDa protein that is missing in four host mutants selected for normal growth after transformation with PKA holoenzyme. A mutant RI cDNA library was generated by PCR random mutagenesis and screened by polycistronic expression in BL21(DE3) cells. The RI cDNA sequence from one revertant has base-pair substitutions creating two amino acid substitutions within the cAMP binding sites. The coexpression of the RI/C alpha subunits in BL21(DE3) bacterial cells provides a system for rapidly selecting mutations in the RI subunits of PKA.